The present invention relates to a method of tuning a local oscillator in a digital broadcast receiver to receive broadcasts that employ quadrature phase-shift keying (hereinafter, QPSK) and orthogonal frequency-division multiplexing (hereinafter, OFDM), and to a digital broadcast receiver using this method.
QPSK-OFDM broadcasts have multiple subcarrier signals, on which data are transmitted in parallel. Each subcarrier signal is modulated separately, then the modulated subcarriers are combined and up-converted to the broadcast frequency to create the QPSK-OFDM broadcast signal.
Reception of a QPSK-OFDM signal requires that a local oscillator in the receiver generate a signal tuned in relation to the broadcast frequency. The local oscillator signal is used to down-convert the received signal to an intermediate-frequency signal, which is then demodulated to obtain the subcarrier phase-shift data. If the local-oscillator frequency is too high or too low, the demodulated phase shifts will be incorrect.
The phase shifts have nominal radian values of zero, .pi./2, .pi., and .pi./2. A conventional method of tuning control multiplies the demodulated phase data by four, modulo 2.pi., so that these nominal values all become zero. The sum of the results for all subcarriers is a phase error signal which can be used to control the local oscillator. Correct tuning is maintained by controlling the frequency of the local oscillator so as to reduce the phase error signal to zero.
The conventional method has the advantage of producing an error value automatically, by a simple and direct calculation, but the error value has an inherent ambiguity of .pi./2. The reason is that multiplication by four converts a phase error of .pi./2 to a phase error of 2.pi., which is the same as a phase error of zero. The conventional method is accordingly limited to correcting small frequency offsets, such as frequency offsets causing phase errors smaller than .pi./4.